Metal halide lamps in widespread use today evolved from pure mercury arc lamps developed early this century. Mercury is an ideal arc generating medium because it is a liquid with a low vapor pressure at room temperature which makes it is relatively easy to strike and sustain an arc. At lamp operating temperatures, the mercury pressure becomes quite high and the voltage across the lamp increases to the point where cost effective, efficient power supplies can be used to drive the lamp. Metal halide salts are added to improve the spectral properties of the mercury discharge thereby making the metal halide lamp more efficacious.
The use of mercury in metal halide lamps presents well known environmental concerns. In response, industry has attempted to provide mercury free lamps. However, previous attempts at lighting systems which include mercury free lamps have presented many problems. For example, some mercury free lighting systems have incurred poor efficacy, color and light characteristics. Moreover, previous mercury free lamps have been restricted to operating conditions wherein the lamp must be positioned horizontally. In particular, the tubular lamps tend to restrict convection and, consequently, chemical mixing in the lamp. When operated horizontally, this is not a problem because the chemicals eventually diffuse uniformly throughout the lamp. However, when the lamp operates vertically, gravity causes the chemicals to collect at the bottom of the lamp. This causes an undesirable color variation along the length of the lamp and a temperature non-uniformity that greatly reduces lamp life. Efforts to improve efficacy and color have met some success yet such improved mercury free lamps can only be operated in a horizontal orientation.
Many efforts have been made to provide a mercury free lamp which overcomes the foregoing shortcomings. For example, U.S. Pat. No. 5,394,057 which issued on Feb. 28, 1995 to Russell et al. illustrates the use of a protective coating on mercury free metal halide lamps.
U.S. Pat. No. 5,363,015 which issued on Nov. 8, 1994 to Dakin et al. relates to a lighting system which includes an electrodeless mercury free metal halide lamp. U.S. Pat. No. 5,479,072 which issued on Dec. 26, 1995 to Dakin et al. is of similar interest.
U.S. Pat. No. 5,523,655 which issued on Jun. 4, 1996 to Jennato et al. illustrates a low pressure lighting system which includes a mercury free neon lamp and a pulsed ballast. U.S. Pat. No. 5,565,741 which issued on Oct. 15, 1996 to Jennato et al., and U.S. Pat. No. 5,666,031 which issued on Sep. 9, 1997 to Jennato et al., are of similar interest.
Notwithstanding the foregoing efforts it is still desired to provide an improved lighting system including a mercury free lamp having photometric results which exceed those heretofore achieved and wherein the lamp may be positioned in any desired orientation. Heretofore, there has been a tendency to design lamps and lamp ballasts separately. As a result, when providing lamp systems, such systems have been optimized individually. The result has often been less than optimal. For example, designers of mercury free lamps have not applied acoustic technology to improve the photometric results of such lamps. The present invention meets the foregoing objectives by providing a lighting system which includes a mercury free metal halide lamp and a ballast system having acoustic arc conditioning.
The use of acoustics in lighting systems is well known. For example, in conventional lamp applications provided heretofore efforts have been made to acoustically influence the arc to straighten or otherwise stabilize and center the arc. For example, U.S. Pat. No. 5,134,345 issued on Jul. 28, 1992 to El-Hamamsy et al. illustrates a method of detecting arc instabilities in a High Intensity Discharge (HID) lamp and changing the drive frequencies that cause them thereby avoiding acoustic frequencies that cause destabilizing phenomena.
In U.S. Pat. No. 5,306,987 issued on Apr. 26, 1994 to Dakin et al. reference is made to stabilization of HID lamps by modulating the drive signal with acoustic resonant band frequencies. A similar method of centering the arc in discharge lamps is illustrated in U.S. Pat. No. 5,198,727 which issued on Mar. 30, 1993 to Allen et al. This patent illustrates centering the arc by the "acoustic perturbations" induced by the frequency of the drive signals. Such acoustic perturbations compel the gas or vapor movement patterns within the arc tube to counter the gravity-induced convection.
U.S. Pat. No. 5,684,367 which issued on Nov. 4, 1997 to Moskowitz et al. illustrates a system for and method of operating a discharge lamp, and in particular, of stabilizing and controlling the characteristics of discharge lamps by amplitude-modulating the input AC power wave with a periodic waveform and/or pulse wave to control stabilization and color characteristics. This patent is commonly owned with the instant application and is incorporated herein by reference.
In U.S. Pat. No. 5,047,695 which issued on Sep. 10, 1991 to Allen et al., a method and ballast circuit is illustrated for operating fluorescent, mercury vapor, sodium and metal halide lamps in a DC mode. Power modulation for creating acoustic pressure waves for arc straightening is referred to in this patent. The lamp illustrated therein is operated with a selectable amount of ripple imposed to provide for acoustically straightening the arc between the lamp electrodes. A related patent is the aforementioned U.S. Pat. No. 5,198,727 patent.
U.S. Pat. No. 5,436,533 issued on Jul. 25, 1995 to Fromm et al. illustrates the operation of a standard metal halide lamp with acoustic stabilization.
All of the foregoing acoustic-related patents relate to straightening or otherwise stabilizing and centering the arc in an arc tube in a discharge lamp.
None of such patents relate to mercury free systems or to the use of acoustic modulation to improve photometric output. In contrast, the present inventors have developed a new lighting method and system using acoustic modulation in the operation of a mercury free discharge lamp to provide improved photometric characteristics regardless of lamp orientation. None of the foregoing references illustrate these features.